1. Field of the Invention
The present invention relates to an angle detection apparatus and a method of estimating an amount of eccentricity of the same.
2. Description of Related Art
A rotary encoder is an angle detection apparatus that detects a rotation angle of a rotation mechanism. The rotary encoder basically includes a circular grating disk and a measurer, for example, the grating disk being etched with a scale pattern including scale marks of several hundreds to several hundred thousands in a radial direction in an external peripheral portion, the measurer being arrayed proximate to the scale pattern of the grating disk and counting passed scale marks as the grating disk rotates. Such a rotary encoder is provided such that the grating disk engages a rotating portion of an object to be measured. The number of scale marks passed by a detector in association with rotation of the object to be measured is counted to detect a rotation angle of the object to be measured.
A rotation axis of the grating disk is, for example, a rolling bearing that rotatably supports the grating disk. The rotation accuracy of the rolling bearing is several ten μm. In general, the rotation axis is centered on the ideal rotation center of the detector and moves along the periphery at a maximum rotation accuracy of a radius. To fix the grating disk and the rotation axis, a jig or the like is used for highly accurate adjustment to match the center of the grating disk and the center of the rotation axis. Thus, the center of the grating disk moves on the same periphery as that of the rotation axis, causing eccentricity relative to the ideal rotation center. Such eccentricity causes an error in a detected angle, such as a change in apparent intervals between the scale marks due to misalignment of the radial position of the scale pattern that the detector reads for angle detection.
A conventional method of eliminating an eccentricity error of a grating disk is disclosed in Japanese Patent No. 4433240, for example. In the method, a grating disk is provided inside a scale pattern with numerous concentric patterns at the same intervals as pitches of the scale pattern. Two detectors are arrayed, which are a measurement detector reading the scale pattern and a correction detector provided at a position rotated by 90° from the measurement detector to read the concentric patterns. Then, the correction detector reads the concentric patterns to correct read signals of the scale pattern generated by the measurement detector, thus eliminating an impact of eccentricity from the read signals of the scale pattern.
The method, however, requires a special grating disk to which numerous concentric patterns are added for correction of eccentricity. Furthermore, with only one detector to read the concentric patterns, the amount of eccentricity of the grating disk eccentric on a two-dimensional plane surface cannot be accurately measured.